Arrangements including image intensifier devices

ABSTRACT

An image intensifier arrangement is operated in a manner which allows observance of preselected events which occur during the occurrence of other events which may mask them. Normally the first stage of the intensifier is operative while the second stage is inoperative. When a preselected event occurs, an electrical pulse circuit switches off the first stage and switches on the second and subsequent stages. The pulse circuit is triggered by a monitoring device which monitors the input signals to the intensifier. The intensifier output may be photographed. In another embodiment, the electrons from the last stage, which may be the second stage, of the intensifier impinge on the target of a television camera tube. The target is scanned by an electron beam and the television signals produced thereby may be displayed and/or recorded for display at a later time.

[ Oct. 23, 1973 1 ARRANGEMENTS INCLUDING IMAGE INTENSIFIER DEVICESRobert Paul Randall, Uxbridge, England EMI Limited, Hayes, Middlesex,England Filed: Nov. 10, 1970 Appl. No.: 88,392

[75] Inventor:

[73] Assignee:

US. Cl. 315/11, 250/213 VT Int. Cl. H01j 31/48 Field of Search 250/213UT, 213 A,

References Cited UNITED STATES PATENTS 2/1970 Pietri et al 250/213 VT7/1961 Farnsworth.. 315/11 12/1969 Anderson.. 315/11 9/1960 Day 315/124/1970 Einstein 315/11 Primary ExaminerCarl D. Quarforth AssistantExaminer-J. M. Potenza Attorney-Fleit, Gipple and Jacobson [57] ABSTRACTAn image intensifier arrangement is operated in a manner which allowsobservance of preselected events which occur during the occurrence ofother events which may mask them. Normally the first stage of theintensifier is operative while the second stage is inoperative. When apreselected event occurs, an electrical pulse circuit switches off thefirst stage and switches on the second and subsequent stages. The pulsecircuit is triggered by a monitoring device which monitors the inputsignals to the intensifier. The intensifier output may be photographed.in another embodiment, the electrons from the last stage, which may bethe second stage, of the intensifier impinge on the target of atelevision camera tube. The target is scanned by an electron beam andthe television signals produced thereby may be displayed and/or recordedfor display at a later time.

7 Claims, 6 Drawing Figures PATENTEDUBI 23 um 3,767.96 1

SHEET 1 BF 3 Invanlor Robgarl Randi ARRANGEMENTS INCLUDING IMAGEINTENSIFIER DEVICES This invention relates to arrangements includingimage intensifier devices having more than one stage. The first stageincludes a photo-cathode and a phosphor screen and, preferably, one ormore focusing electrodes. The second stage includes a photo-cathode, andone or more focusing electrodes which cause the electrons from thephoto-cathode to be focused to a new image plane. Such devices areherein referred to as devices of the kind described.

Such an arrangement is suitable for intensifying preselected events ofshort duration especially when they occur during the occurrence ofevents of longer duration, or for events of a preselected energy levelespecially when they occur during the occurrence of events having awider band of energies, which may tend to mask the preselected events.

The electrons from the second stage may then be utilized in any suitablemanner. For example they may be utilized in further stages of imageintensification; or they can be used to react with various forms ofelectron sensitive targets, such as storage targets. This type ofselection is desirable in, for example, gamma ray imaging wherein gammarays are directed onto a scintillating crystal and then viewed by animage intensifier device. The rays incident on the intensifier devicemay consist of rays of different energy levels, only a small range ofwhich corresponds to the image, the others resulting from scattering ofthe gamma rays or from other processes which are not related to theimage of interest.

It is an object of the invention to provide an arrangement which allowsintensification and display of selected events.

According to the invention there is provided an arrangement comprisingan image intensifier device and an electric circuit, said deviceincluding an input photocathode, first focusing electrode means; anintensifying electrode, second focusing electrode means and a receivingelectrode for receiving electrons from said intensifying electrode, saidcircuit including means for applying respective substantially constantpotentials to said input photocathode, said second focusing electrodemeans and said receiving electrode and means for applying respectivepotentials to said first focusing electrode means and said intensifyingelectrode such as to allow input signals received on said inputphotocathode to be transmitted to said intensifying electrode whilepreventing signals from being transmitted from said intensifyingelectrode to said receiving electrode, said circuit also includingmonitoring means for monitoring the input signals to said inputphotocathode and, when a signal occurs having a desired characteristic,for causing a voltage pulse to be applied to each of said first focusingelectrode means and said intensifying electrode whereby said signalhaving a desired characteristic is intensified and transmitted to saidreceiving electrode while further input signals are prevented from beingtransmitted from said input photocathode to said intensifying electrode,said intensifying electrode having sufficient storage to allow time forsaid monitoring means to detect an input signal having adesiredcharacteristic and the subsequent application of said voltage pulses.

In order that the invention may be clearly understood and readilycarried into effect it will now be described by way of example withreference to the accompanying drawings in which:

FIG. 1 illustrates diagrammatically an arrangement according to oneexample of the invention,

FIGS. 2a to 2d exemplify the principle of the operation of thearrangement shown in FIG. 1, and

FIG. 3 illustrates diagrammatically an arrangement according to anotherexample of the invention.

Referring to FIG. 1 which illustrates one example of the invention, a4-stage image intensifier tube has an insulating envelope 1 and inputand output windows 2 and 3 respectively. Input window 2 supports aphotocathode layer 5 while output window 3 supports a phosphor layer 7.Spaced along the tube are intensifying electrodes D1 to D3 and ringfocusing electrodes 8 to 15. The electrodes D1 to D3 are of the knownsandwich construction having a phosphor layer facing the input end ofthe tube and a photo-cathode layer facing the output end of the tube.The phosphor layer of electrode D1 exhibits lag, that is it has astorage effect, the decay characteristic being chosen dependent upon thefrequency of the events it is required to select. The material may befor example titanium-activated zinc silicate which has a decay period,and hence a storage period, of 5p. sec.

The principle of operation will now be described with reference to FIGS.2a to 2d. FIG. 2a shows how the potentials may be applied to theelectrodes of the tube by means of a divider chain connected between anEHT supply and earth while FIG. 2b illustrates the valves in KV whichthese potentials may have. With these potentials applied to theelectrodes, all four stages would be operative, that is to say an imagereceived on input photo-cathode 5 would be transmitted down the tube,intensified by electrodes D1 to D3 and displayed on output phosphor 7.FIG. 20 indicates the potentials in RV applied to the electrodes for thetube to operate in a receive only mode, that is with the first stageoperative and subsequent stages unresponsive to the output of the firststage (hereinafter referred to as the first state of the tube). It willbe seen that the potential on electrode D1 exceeds by 500V the potentialon electrode 10, which is the first electrode in the second stage, sothat the second stage is inoperative and the image received by thephosphor of electrode D1 is stored but not transmitted, the storageperiod depending upon the material of this phosphor. The potential onelectrode D1 has thus been raised, and in order that the first stageshall remain operative and in a focused state the potentials onphoto-cathode 5 and electrodes 8 and 9 are also raised by the sameamount as electrode D1, in this case 3.5KV. FIG. 2d illustrates thepotentials applied to the electrodes of the tube for the transmissionmode, that is with the first stage inoperative and the remaining stagessimultaneously operative (hereinafter referred to as the second state ofthe tube). It will be seen that the potential applied to electrode D1has been reduced by 3.5KV to its previous value of 9KV so that thesecond and subsequent stages now transmit and intensify the image storedon the phosphor of electrode D1. The potential on electrode 8 has alsobeen reduced by 3.5KV so that the potential thereon is now 500V belowthe potential on the input photo-cathode 5 and thus the first stage isinoperative during this transmission period. Further, the potential onelectrode 9 has also been reduced by 3.5KV as it is found that transientchanges in potential between electrodes 8, 9

and D1 can cause severe spurious signals to occur in the first stagewhich would normally break through and be intensified with the storedimage, thus possibly resulting in serious noise conditions in the outputimage. By allowing electrodes 8, 9 and D1 to maintain their relativepotential differences, this cause of spurious signals is eliminated.

It will be seen from the preceding paragraph on the principle ofoperation that the potentials on only three electrodes requires to bevaried, namely electrodes 8, 9 and D1, as shown in FIGS. 2c and 2d. Inpractice, these changes of potential are obtained by pulsing techniques.

Referring again to FIG. 1, the potentials for photocathode 5 andelectrodes 8, 9 and D1 are obtained independently of the potentials forthe remaining electrodes. To ease circuit problems in pulsing the tube,the potential on electrode 8 is in the region of the earth point and thephoto-cathode 5 is run at 3.3KV from voltage source EHT3. The potentialsfor electrodes 8, 9 and D1 are obtained from a 7KV voltage source EHT2and associated resistor chain, and the potentials for the remainingelectrodes are obtained from a 33KV voltage source EHTl and a dividerchain. The polarities of the three EHT sources are as indicated by theand signs on the leads adjacent thereto. The divider chain associatedwith voltage source El-lTl comprises a plurality of resistors togetherwith three voltage controllers VC1, VC2 and VC3 which are manuallyadjustable in the manner of a fine tuner to provide a voltage variationof about 3 percent. So that the cut-off conditions in the second stagemay be independent of the focusing potential applied to electrode D2,the potentials for the second stage electrodes 10 and 11 are obtainedfrom the centre tap of a 500M!) resistor chain connected across VC2.

The circuit described thusfar operates the tube in its first state, ashereinbefore defined. In this mode, the image on input photo-cathode 5is transmitted to the phosphor layer of electrode D1 where it is stored.As mentioned previously, this storage period depends upon the materialof the phosphor layer, and in the case of titanium-activated zincsilicate it is about 5,u. sec. The storage period of the phosphor layerof electrode D1 is largely responsible for the repetition rate which canbe employed, that is the frequency of the events which can beindividually intensified and displayed, and in the case oftitanium-activated zinc silicate the maximum useful repetition rate isabout I0 per see. If higher rates are required, then phosphors having ashorter decay time can be used, for example P16 or P24 phosphorsaccording to the JEDEC classification.

As previously mentioned, the necessary changes in the potentials appliedto the electrodes are achieved by means of pulses. The pulsing circuitoperates in known manner and includes input terminals A and B to whichtriggering signals are applied from a monitoring device M which may be aphoto-electric device, for example a photomultiplier tube, upon theoccurrence of an event of interest. However, triggering signals may beobtained from any suitable means which operate sufficiently fast toallow the tube to be pulsed from its first state to its second statebefore the image on the phosphor of electrode D1 has decayed. Themonitoring device may be arranged to produce a triggering pulse by, forexample, pulse height analysis of the output of the photomultiplier orother means, a triggering pulse being produced only when the pulseheight exceeds a preselected threshold. On the other hand if the timesof the events are known then a pulse generator operating at appropriatetimes may be employed. The pulsing circuit comprises essentially apentode V1 in a common cathode stage which drives a cathode follower V2which is directly coupled to the control grid of a pulse tetrode V3. Thecommon cathode stage provides theoretical voltage gain of approximately60 but is overdriven (in order to square off the waveform) by thetriggering signal which is 10V negative. This provides a 180V positivesignal to feed the cathode follower which is used as a low impedancedriver for the pulse tetrode V3, producing a pulse of :l20V amplitudehaving a rise time of 0.25 1. sec and a fall time of 1.511 sec. Thecontrol grid of pulse tetrode V3 is held at -l00V via a 2.2K!) resistorand is switched well into conduction when the V pulse drives it intogrid current. The tetrode is operated in the grid current region toensure that it conducts as heavily as possible so as to reduce the risetime and ensure that the anode saturation voltage is kept low so as notto introduce a possible variable. When the tetrode is switched on, the0.08 11 reservoir capacitor discharges through the tetrode to produce alarge negative voltage pulse which is applied to electrodes 8, 9 and D1.The amplitude of this pulse is determined by the ratio Rl/(Rl+R2) withan additional 2K9 resistor and a 2K!) potentiometer included to allow asmall variation. R1 and R2 are about 5K0. and 4K!) respectively, theprecise value of the ratio being chosen so that the potentials onelectrodes 8, 9 and D1 are pulled down sufficiently for the tube to beswitched from the first state to the second state (as hereinbeforedefined). After transmitting the image stored on the phosphor layer ofelectrode DI, the tube returns to its first state to await theoccurrence of the next event of interest.

If desired, the displayed images may be photographed.

The maximum framing rate when all the phosphors are silver-activatedzinc sulphide is about 500 to 1,000 per sec. The framing rate can beincreased by using phosphors having a shorter decay time. For example,employing titanium-activated zinc silicate for all the phosphors allowsa maximum framing rate of 30,000 per sec. In this case however, themaximum tube gain will be between 10 and 4 X 10, as compared with about10 due to the lower efficiency of zinc silicate as compared with zincsulphide. Where photography is used to record the output of the tube,long exposure times can be used to capture a random event, as the signaldisplayed on phosphor 7 while the tube is in the first state will beonly that due to the dark currents of the photocathodes of electrodes D2and D3. This is unlikely to raise the fog level of even fast films forexposures of many minutes, as very low dark current bialkaliphotocathodes (antimony-sodium-potassium) may be used in these stages ofthe tube.

It may be desirable in certain applications to incorporate an additionalcircuit to limit the pulse repetition frequency, thereby protecting thepulsing circuit from excessive power dissipation. This additionalcircuit may suitably include a limiter comprising a triode connectedacross the grid leak of pentode V2 driven by a monostable which is fedfrom a diode pump staircase counter with sufficient leakage just to holdoff at the required pulse repetition frequency, say 500 per sec.

The monostable could be designed to have an OFF recovery time of 0.1 secso as not to cause undue loss of picture information whilst stillaffording a certain degree of protection to the pulsing circuit.

Referring now to FIG. 3, in which similar reference numerals denotesimilar parts to those shown in FIG. 1, an envelope 1 has an inputwindow 2 which carries a photo-cathode 5. Focusing electrodes 8 and 9focus electrons from photo-cathode 5 onto an intensifying electrode D1of the known sandwich type having a phosphor layer on the side facingthe input window 2 and a photo-cathode layer on the side remotetherefrom. Electrons from the photo-cathode layer of electrode D1 arefocused by focusing electrodes 10 and 11 onto a target T of a televisioncamera tube system. The camera tube system has an electron gun structureG, and pins P for making electrical connections thereto are hermeticallysealed through the envelope 1. In the form shown, the camera tube is ofthe all electrostatic type but it will be appreciated that magneticfocusing and/or deflection may be employed if desired. The target T maybe of any suitable electron sensitive type. For example, it may comprisea material which exhibits electron bombardment induced conductivity ormay comprise a two dimensional array of semiconductor diodes. Inoperation, the first stage of the intensifier is normally operative andthe second stage is inoperative. When an event of interest having adesired characteristic occurs, the first stage of the intensifier isswitched to be inoperative and the second stage is switched to beoperative. The electrons from the photo-cathode layer of electrode D1are focused onto the target T which is then scanned by electrons fromthe electron gun G. The television signals thereby produced may bedisplayed and/or recorded for display at a later time. In a furtherarrangement, a camera tube may be associated in like manner with animage intensifier having three or more stages. In this case, the secondand subsequent stages of the intensifier may be switched to besimultaneously operative when a preselected event occurs. The potentialsfor the electrodes of the image intensifier may be derived and switchedby means similar to those described with reference to FIG. 1.

An arrangement according to the invention thus allows preselected eventshaving a desired characteristic to be observed, and a variety ofterminal devices may be employed depending upon the use to be made ofthe information concerning these events.

Various modifications will be apparent to anyone skilled in the art. Forexample, the sandwich type intensifying electrodes D2 and/or D3 may bereplaced by other intensifying types of electrodes such as transmissivesecondary electron emissive types. Also, the target T may be made of amaterial exhibiting secondary electron emission.

What I claim is:

I. An arrangement comprising an image intensifier device and an electriccircuit, said device including an input photocathode, first focusingelectrode means, an intensifying electrode, second focusing electrodemeans and a receiving electrode for receiving electrons from saidintensifying electrode, said circuit including means for applyingrespective substantially constant potentials to said input photocathode,said second focusing electrode means and said receiving electrode andmeans for applying respective potentials to said first focusingelectrode means and said intensifying electrode such as to allow inputsignals received on said input photocathode to be transmitted to saidintensifying electrode while preventing signals from being transmittedfrom said intensifying electrode to said receiving electrode, saidcircuit also including monitoring means for monitoring the input signalsto said input photocathode and, when a signal occurs having a desiredcharacteristic, for causing a voltage pulse to be applied to each ofsaid first focusing electrode means and said intensifying electrodewhereby said signal having a desired characteristic is intensified andtransmitted to said receiving electrode while further input signals areprevented from being transmitted from said input photocathode to saidintensifying electrode, said intensifying electrode having sufficientstorage to allow time for said monitoring means to detect an inputsignal having a desired characteristic and the subsequent application ofsaid voltage pulses.

2. An arrangement according to claim 1 in which said receiving electrodecomprises the target of a television camera tube.

3. An arrangement according to claim 1 in which said receiving electrodecomprises a further intensifying electrode.

4. An arrangement according to claim 1 wherein said monitoring meanscomprises a photo-electric device.

5. An arrangement according to claim 4 wherein said photo-electricdevice comprises a photomultiplier.

6. An arrangement according to claim 2 wherein said target comprises anarray of semiconductor diodes.

7. An arrangement according to claim 2 wherein said target comprises amaterial which exhibits electrons bombardment induced conductivity.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 a767'v D d October 23 1973 Inventor) Robert Paul Randall It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

On the title page, insert [30] Foreign Application Priority Data Nov;19, 1969 Great Britain 56531/69 Signed and sealed this 18th day of June19711.. v

(SEAL). Attest: mm M'.FLETCHER,JR. c. MARSHALL 1mm Atteating OfficerCommissioner of Patents DRM PO-IOSO (10-69) USCOMM-DC 6O876-P69 u.s.GOVERNMENT mm'rmg ornc: "I! 0-;56-334.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 IDated October 23 1973 Inventor-(s) Robert Paul Randall It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below! On the title page, insert[30] Foreign Application Priority Data Nov; 19', 1969 Great Britain56531/69 Signed and sealed this 18th day of June 1971;; 1

(SEAL) 'Attest: EWARD MrFLETCHERJ'R. c. MARSHALL 1mm Atteating OfficerCommissioner of Patents )RM PO-1050 (10-69)

1. An arrangement comprising an image intensifier device and an electriccircuit, said device including an input photocathode, first focusingelectrode means, an intensifying electrode, second focusing electrodemeans and a receiving electrode for receiving electrons from saidintensifying electrode, said circuit including means for applyingrespective substantially constant potentials to said input photocathode,said second focusing electrode means and said receiving electrode andmeans for applying respective potentials to said first focusingelectrode means and said intensifying electrode such as to allow inputsignals received on said input photocathode to be transmitted to saidintensifying electrode while preventing signals from being transmittedfrom said intensifying electrode to said receiving electrode, saidcircuit also including monitoring means for monitoring the input signalsto said input photocathode and, when a signal occurs having a desiredcharacteristic, for causing a voltage pulse to be applied to each ofsaid first focusing electrode means and said intensifying electrodewhereby said signal having a desired characteristic is intensified andtransmittEd to said receiving electrode while further input signals areprevented from being transmitted from said input photocathode to saidintensifying electrode, said intensifying electrode having sufficientstorage to allow time for said monitoring means to detect an inputsignal having a desired characteristic and the subsequent application ofsaid voltage pulses.
 2. An arrangement according to claim 1 in whichsaid receiving electrode comprises the target of a television cameratube.
 3. An arrangement according to claim 1 in which said receivingelectrode comprises a further intensifying electrode.
 4. An arrangementaccording to claim 1 wherein said monitoring means comprises aphoto-electric device.
 5. An arrangement according to claim 4 whereinsaid photo-electric device comprises a photomultiplier.
 6. Anarrangement according to claim 2 wherein said target comprises an arrayof semiconductor diodes.
 7. An arrangement according to claim 2 whereinsaid target comprises a material which exhibits electrons bombardmentinduced conductivity.